Solder ball fabricating process

ABSTRACT

A solder ball fabricating process for forming solder balls over a wafer having an active layer is provided. A patterned solder mask layer is formed over the active surface of the wafer. The patterned solder mask layer has an opening that exposes a bonding pad on the wafer. Solder material is deposited into the opening over the bonding pad. A reflow process is conducted to form a pre-solder body. The aforementioned steps are repeated so that various solder materials are fused together to form a solder ball over the bonding pad.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of a prior application Ser. No.10/248,863, filed Feb. 26, 2003, now U.S. Pat. No. 6,673,711, whichclaims the priority benefit of Taiwan application Ser. No. 91103530,filed on Feb. 27, 2002.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a solder ball fabricating process. Moreparticularly, the present invention relates to a solder ball fabricatingprocess for the fabrication of a wafer-level chip scale package (WLCSP).

2. Description of Related Art

Due to the trend of developing light and compact electronic products,the size of most integrated circuit packages continues to decrease. Toreduce the size of integrated circuit (IC) packages, chip scale packages(CSP) have been developed. In general, the edge length of a CSP packageis roughly 1.2 times the edge length of a silicon chip or thechip/package has an area ratio of about 80% and the pitch between leadsis limited to a value under 1 mm. Many types of chip scale packages arenow available. However, the most common type is one having the packagedirectly formed on the wafer, known also as a wafer-level chip scalepackage (WLCSP).

One major characteristics of a WLCSP is the fabrication of aredistribution circuit (RC) on the surface of the chip so that thebonding pads originally positioned around the periphery of the chip areredistributed as an area array on top of the chip. Hence, the entiresurface of the chip can be utilized for accommodating bonding pads,thereby producing a larger pitch between bonding pads to meet the largerdistance of separation between contacts on a printed circuit board(PCB). In addition, solder balls are attached to the bonding pads of thechip manually or automatically so that the bonding pads on the chip areelectrically connected to the contacts on the PCB through the solderballs.

However, if the positions of the original bonding pads and pitch betweenthe original bonding pads on the chip match the contact pitch in theprinted circuit board, there is no need to form the redistributioncircuit on the chip. In other words, the solder balls may be directlyattached to the original bonding pads on the chip. In the followingdescription, the solder ball pads refers to all the bonding pads on achip requiring solder ball attachment, for example, including theoriginal bonding pad on the chip or the bonding pads on theredistribution circuit above the chip.

As integrated circuit design progresses and the level of integrationcontinues to increase, the number of output pads in a chip alsoincreases. Yet, surface area of the chip often remains identical or isreduced slightly. Under such circumstances, the conventional solder ballattachment technique can hardly accommodate fine solder balls.Ultimately, small fine pitch solder balls have to be used in thefabrication of WLCSP.

Furthermore, the conventional solder ball attachment technique can beroughly classified into the automatic ball attachment method and themanual ball attachment method. The automatic ball attachment methodcosts more to operate especially for attaching small fine pitch solderball. Although the manual ball attachment method is less expensive tooperate, substantial labor force is required and overall ball attachmentefficiency is relatively low. Since it is difficult to attach small finepitch solder balls to the bonding pad of a chip in a WLCSP, a largersize bump is often attached to the bonding pad of the chip instead of asolder ball.

Because lead-tin alloy has a good bonding strength as well as physicaland conductive properties, lead-alloy is often used as a solder materialfor joining devices on a chip with contacts on the printed circuit boardin the fabrication of integrated circuit packages. However, lead is atoxic material that often causes health hazards and environmentalconcerns. Thus, the electronic industry is actively looking for alead-free substitute for the lead-containing solder material. Atpresent, a number of lead-free solder materials have already beendeveloped. In the not too distant future, all lead-containing soldermaterial will be replaced.

SUMMARY OF INVENTION

Accordingly, one object of the present invention is to provide a solderball fabricating process for directly forming a solder ball on thebonding pad of a wafer in a wafer level chip scale package (WLCSP). Thesolder ball fabricating process not only increases production rate, butthe size and height of the solder ball is also much easier to controlwithin the desired range. In addition, constituents inside a lead-freesolder ball are easier to control when the invention is applied to formlead-free solder balls.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a solder ball fabricating process for attaching atleast one solder ball to a wafer. The wafer has an active surface, apassivation layer and at least one bonding pad. The passivation layerand the bonding pad are formed on the active surface of the wafer suchthat the passivation layer exposes the bonding pad. The wafer furtherincludes a stress buffer layer and at least one under-ball-metallurgylayer. The under-ball-metallurgy layer is formed over the bonding pad.The stress buffer layer is formed over the passivation layer but alsoexposes the under-ball-metallurgy layer. First, a patterned first soldermask layer is formed over the stress buffer layer. The first solder maskhas at least one first opening that exposes the under-ball-metallurgylayer. A first solder material is deposited into the first opening andthen a first reflow process is carried out so that the first soldermaterial inside the first opening is turned into a pre-solder body.Thereafter, a patterned second solder mask layer is formed over thefirst solder mask layer. The second solder mask layer has at least asecond opening located above the first opening and exposing thepre-solder body. The second opening also has a diameter greater than thefirst opening. A second solder material is deposited into the secondopening and then a second reflow process is carried out so that thesecond solder material inside the second opening and the pre-solder bodymelt together to form a solder ball above the under-ball-metallurgylayer. Finally, the first solder mask layer and the second solder masklayer are removed.

The solder ball fabricating process according to this invention includesforming a solder mask layer over the wafer and patterning the soldermask layer to form an opening. The opening exposes theunder-ball-metallurgy layer above the bonding pad of the wafer.Thereafter, a solder material is deposited into the opening so that thesolder material stacks on top of the under-ball-metallurgy layer. Areflow process is conducted next to melt the solder material into apre-solder body. The aforementioned steps are repeated once so thatvarious solder materials are melted into a solder ball above the bondingpad. Note that diameter of the opening in each solder mask layer may notbe the same. Hence, a staircase-like or an inverted frustum-cone-likecavity structure is formed. This type of cavity structure facilitatesthe deposition of solder material into the opening. Therefore, solderball having greater size and height are formed over theunder-ball-metallurgy layer after the reflow of various solder materialsis completed.

Similarly, the solder ball fabricating process according to thisinvention can be applied to fabricate lead-free solder balls. Since theconstituents of a lead-free solder ball include metallic substances oralloys combined in various ratios, the lead-free solder balls are formedby sequentially stacking various types of solder materials over thebonding pads of the wafer and then melting the solder materialstogether. By adjusting the thickness of each solder mask layer and sizeof each opening, volume of solder material deposited into each openingcan be precisely adjusted so that the ultimately formed lead-free solderball has the desired height and contains all the necessary constituentsmixed in the desired ratio.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIGS. 1 to 8 are schematic cross-sectional views showing the stepscarried out in a solder ball fabricating process according to onepreferred embodiment of this invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIGS. 1 to 8 are schematic cross-sectional views showing the stepscarried out in a solder ball fabricating process according to onepreferred embodiment of this invention. As shown in FIG. 1, a waferhaving an active surface 12, a passivation layer 14 and a plurality ofsolder ball pads 16 (only one is shown) is provided. The passivationlayer 14 and the solder ball pads 16 are formed on the active surface 12of the wafer such that the passivation layer 14 exposes the solder ballpad 16. The solder ball pad 16 refers to any bonding pads or connectionpads on the wafer 10. The wafer 10 further includes a plurality ofunder-ball-metallurgy (UBM) layers 18 (only one is shown) and a stressbuffer layer 20. The under-ball-metallurgy layer 18 is formed over thesolder ball pad 16 and the stress buffer layer 20 is formed over thepassivation layer 14. The stress buffer layer 20 also exposes theunder-ball-metallurgy layer 18.

The wafer 10 may be packaged according to a chip scale package (CSP) sothat a chip cut out from a wafer is mounted onto a carrier. Damages tothe chip and other package structures may occur due to thermal stressresulting from differences in the coefficient of thermal expansion (CTE)between the chip and the carrier. The stress buffer layer 20 is used tobuffer the thermal stress between the chip and the carrier so thatstructural damage between the chip and the carrier is minimized. Thestress buffer layer is commonly fabricated using a material such asbenzocyclobutene (BCB).

The under-ball-metallurgy layer 18 is designed to increase the bondingstrength between a solder ball 34 and the solder ball pad 16 as shown inFIG. 7. In the meantime, the under-ball-metallurgy layer 18 also servesto prevent the inter-diffusion of metallic elements and avoid theoxidation of metallic layers. The under-ball-metallurgy layer 18 isusually formed before the stress buffer layer 20. Photolithographicprocess together with evaporation, sputtering or electroplatingprocesses are often used to fabricate the under-ball-metallurgy layer18. An alternative method of forming the under-ball-metallurgy layer 18is to form an under-ball-metallurgy layer over the active surface 12 ofthe wafer 10 globally and performing photolithographic and etchingprocess in sequence to pattern the under-ball-metallurgy layer.Furthermore, the under-ball-metallurgy layer typically includes a stackof metallic layers such as a wetting layer, a barrier layer and anadhesion layer. These metallic layers are fabricated using a single typeof metal such as copper, chromium, titanium, tungsten, silver, nickel,vanadium and aluminum or an alloy of some of the above metals.

As shown in FIG. 2, a patterned first solder mask layer 22 is formedover the stress buffer layer 20. The patterned first solder mask layer22 has a plurality of first openings 24 (only one is shown). Thepatterned first solder mask layer 22 is formed in a lamination process,for example. In the lamination process, a photosensitive dry film isadhered to the surface of the stress buffer layer 20. Alternatively, thepatterned first solder mask layer 22 is formed by spin-coating liquidphotoresist on the stress buffer layer 20. Thereafter, using a photo-viamethod, the photosensitive dry film or the photoresist layer ispatterned to form a first opening 24 in the first solder mask layer 22.Thickness of the first solder mask layer 22 determines the height of thefirst opening 24.

As shown in FIG. 3, a printing method is applied to deposit a first typeof solder material 26 into the first opening 24. The first soldermaterial in solder powder form or solder paste form accumulates over theunder-ball-metallurgy layer 18 inside the first opening 24.

As shown in FIG. 4, a first reflow process is conducted so that thefirst solder material 26 is transformed into a pre-solder body 28 on topof the under-ball-metallurgy layer 18.

As shown in FIG. 5, a patterned second solder mask layer 30 is formedover the first solder mask layer 22. The second solder mask layer has aplurality of second openings 32 (only one is shown) that exposescorresponding pre-solder body 28. Since the second solder mask 30 isformed and patterned in way similar to the first solder mask 22,detailed description of the steps is omitted here.

As shown in FIG. 6, a printing method is similarly applied to deposit asecond type of solder material 34 into the second opening 32. The secondsolder material 34 also in solder powder form or solder paste formaccumulates over the pre-solder body 28 (formed from the first soldermaterial 26) inside the second opening 32. The second solder material 34may contain the same material as the pre-solder body 28 (that is, thefirst solder material 26) or different constituents. Note that thesecond opening 32 has a diameter larger than the first opening 24.Consequently, the first opening 24 together with the second opening 32form a staircase-like or inverted frustum-cone-like cavity structurethat facilitates the deposition of the second solder material 34.

As shown in FIG. 7, a second reflow process is carried out to melt andfuse the pre-solder body 28 and the second solder material 34 togetherand form a bigger solder ball 36 over the under-ball-metallurgy layer18. Finally, the first solder mask layer 22 and the second masking layer30 are removed to expose the solder ball 36 to form a structure shown inFIG. 8.

The solder ball fabricating process according to this invention can alsobe applied to the fabrication of lead-free solder balls as shown inFIGS. 6 and 7. For example, if pre-solder body 28 (the first soldermaterial 26) is a tin-silver alloy (95 Sn/5 Ag) and the second soldermaterial 34 is tin, the solder ball 36 after a reflow process is atin-rich tin-silver alloy solder ball. On the other hand, if thepre-solder body 28 (the first solder material 26) is copper and thesecond solder material 34 is tin, the solder ball 36 after a reflowprocess is a tin-copper alloy solder ball. If the pre-solder body 28(the first solder material 26) is silver and the second solder material34 is tin, the solder ball 36 after a reflow process is a tin-silveralloy solder ball. Similarly, if pre-solder body 28 (the first soldermaterial 26) is a tin-silver alloy (95 Sn/5 Ag) and the second soldermaterial 34 is copper, the solder ball 36 after a reflow process is atin-silver-copper alloy solder ball.

Similarly, as shown in FIGS. 6 and 7, the solder ball 37 on the wafer 10has a diameter of about 400 μm. To form a solder ball having such asize, an under-ball-metallurgy layer 18 having a diameter of about 150μm, a first opening 24 having a width of about 700 μm and a secondopening 32 having a width of about 900 μm, for example, are fabricated.

The solder ball fabricating process according to this invention includesforming a patterned solder mask layer over the wafer. The patternedsolder mask layer contains openings that expose solder ball pads on thewafer. A solder material is then deposited into the openings and areflow process is carried out to form pre-solder body inside eachopening. A second patterned solder mask layer having openings therein isformed over the first solder mask layer. The second openings expose theoriginal pre-solder body. Thereafter, a second type of solder materialis deposited into the second opening to accumulate over the pre-solderbody. A second reflow process is conducted to melt and fuse the twotypes of solder materials into a single solder ball over the solder ballpad (or the under-ball-metallurgy layer).

Aside from forming two patterned solder mask layers, this invention alsopermits a repetition of the aforementioned steps to form more patternedsolder mask layers. Each repetition includes forming a patterned soldermask layer, filling the opening with solder material and conducting areflow process to form a pre-solder body. Finally, the stack of soldermaterials are melted and fused together to form a solder ball over thesolder ball pad (the under-ball-metallurgy layer).

This invention also utilizes the openings in various solder mask layersto form a staircase-like or inverted frustum-cone-like cavity structurethat facilitates the deposition of solder material and providessufficient support to present the collapse of the pre-solder body duringa reflow process. Hence, size and height of the solder ball can besignificantly increased to meet design criteria.

In summary, the solder ball fabricating process according to thisinvention includes forming a solder mask layer on the active surface ofa wafer and patterning the solder mask layer to form an opening. Theopening exposes the under-ball-metallurgy layer above the solder ballpad of the wafer. Thereafter, a solder material is deposited into theopening and a reflow process is conducted next to melt the soldermaterial into a pre-solder body. The aforementioned steps are repeatedonce so that various solder materials are melted into a solder ballabove the bonding pad. Since the openings in various solder mask layerstogether form a staircase-like or an inverted frustum-cone-like cavitystructure, a wider area for accepting the deposition of solder materialis provided. Thus, solder balls having a greater size and height can beproduced over the under-ball-metallurgy layer to meet various designs.

In addition, the solder ball fabricating process of this invention canbe used to form solder bumps over a wafer. In this case, the soldermaterial deposited into the openings must have good solderingproperties. Due to environmental and health considerations,lead-containing solder material (such as lead-tin alloy) is likely tophase out soon. Ultimately, lead-free solder ball will have to befabricated.

The solder ball fabricating process according to this invention can beapplied to the fabrication of lead-free solder balls. Since theconstituents of a lead-free solder ball include metallic substances oralloys combined in various ratios, the lead-free solder ball is formedby sequentially stacking various types of solder materials over thebonding pads of the wafer and then melting the solder materialstogether. By adjusting the thickness of each solder mask layer and sizeof each opening, the volume of solder material deposited into eachopening can be precisely adjusted so that the ultimately formedlead-free solder ball has the desired height and contains all thenecessary constituents mixed in the desired ratio.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A solder ball fabricating process for producing at least one solderball on a wafer, the method comprising: forming a structured layer overthe wafer, wherein the structure layer includes at least anunder-ball-metallurgy layer being exposed; forming a first patternedsolder mask layer over the structured layer, wherein the first patternedsolder mask layer has at least a first opening that exposes theunder-ball-metallurgy layer; depositing a first solder material into thefirst opening; conducting a first reflow process so that the firstsolder material melts to form a pre-solder body over theunder-ball-metallurgy layer; forming a second patterned solder masklayer over the first patterned solder mask layer, wherein the secondpatterned solder mask layer has at least one second openingcorresponding to the first opening to expose the pre-solder body and thesecond opening is larger than the first opening in an opening size;depositing a second solder material into the second opening; conductinga second reflow process so that the pre-solder body and the secondsolder material melt and fuse together to form a solder ball over theunder-ball-metallurgy layer; and removing the first solder mask layerand the second solder mask layer.
 2. The process of claim 1, wherein thefirst solder material and the second solder material are identical. 3.The process of claim 1, wherein the first solder material is differentfrom the second solder material.
 4. The process of claim 1, wherein thefirst solder material has a melting point higher than that of the secondsolder material.
 5. The process of claim 1, wherein the solder ball is alead-containing solder ball.
 6. The process of claim 1, wherein thesolder ball is a lead-free solder ball.
 7. The process of claim 1,wherein the first patterned solder mask layer includes a photoresistlayer.
 8. The process of claim 1, wherein the first patterned soldermask layer includes a dry film.
 9. The process of claim 1, wherein thestep of forming the second patterned solder mask layer includeslaminating a mask layer over the first patterned solder mask layer andthen patterning the second solder mask layer.
 10. The process of claim1, wherein the step of forming the second patterned solder mask layerincludes spin-coating a mask layer over the first patterned solder masklayer and then patterning the second solder mask layer.
 11. The processof claim 1, wherein the first solder material is a solder powder or asolder paste.
 12. The process of claim 1, wherein the second soldermaterial is a solder powder or a solder paste.
 13. The process of claim1, wherein the step of forming the structured layer comprises forming aredistribution layer to redistribute a bond pad of a chip to theunder-ball-metallurgy layer.
 14. The process of claim 1, wherein theunder-ball-metallurgy layer is disposed and electrically connected to abond pad of a chip.
 15. A solder ball fabricating process for producingat least one solder ball on a wafer, the method comprising: forming astructured layer over the wafer, wherein the structure layer includes atleast an under-ball-metallurgy layer being exposed; forming a firstpatterned solder mask layer over the structured layer, wherein the firstpatterned solder mask layer has at least a first opening that exposesthe under-ball-metallurgy layer; forming a second patterned solder masklayer over the first patterned solder mask layer, wherein the secondpatterned solder mask layer has at least one second openingcorresponding to the first opening and the second opening is larger thanthe first opening in opening size; depositing a first solder materialinto the first opening; depositing a second solder material into thesecond opening; conducting a reflow process on the foregoing soldermaterials within the forgoing openings to form a solder ball over theunder-ball-metallurgy layer; and removing the forgoing solder masklayers.
 16. The process of claim 15, wherein before the step of formingthe second patterned solder mask layer, the step of depositing the firstsolder material into the first opening is performed and a pre-reflowprocess is performed on the first solder material.
 17. The process ofclaim 15, wherein the step of forming the structured layer comprisesforming a redistribution layer to redistribute a bond pad of a chip tothe under-ball-metallurgy layer.
 18. The process of claim 15, whereinthe under-ball-metallurgy layer is disposed and electrically connectedto a bond pad of a chip.
 19. The process of claim 15, wherein the firstsolder material is different from the second solder material.
 20. Theprocess of claim 15, before the step of conducting the reflow process,further comprising: forming a third patterned solder mask layer with athird opening over the second patterned solder masks, wherein the thirdopening is aligned to the second opening and is larger in open size thanthe second opening; and depositing a third solder material into thethird opening.